This invention relates to multilayer ceramic substrates, and more particularly relates to multilayer ceramic substrates useful for electronics packaging and to a method for making such substrates.
Glass, ceramic and glass ceramic (hereinafter just ceramic) structures, usually and preferably multilayered, are used in the production of electronic substrates and devices. Many different types of structures can be used, and a few of these structures are described below. For example, a multilayered ceramic circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as insulators. The substrates may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, covers, etc. Interconnection between buried conductor levels can be achieved through vias formed by metal paste-filled holes in the individual ceramic layers formed prior to lamination, which, upon sintering will become a sintered dense metal interconnection of metal based conductor.
In general, conventional ceramic structures are formed from ceramic green sheets which are prepared by mixing a ceramic particulate, a thermoplastic polymeric binder, plasticizers and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible green sheets. After blanking, via formation, stacking and laminating, the green sheet laminates are eventually fired at temperatures sufficient to drive off the polymeric binder resin and sinter the ceramic particulates together into a densified ceramic substrate.
The electrical conductors used in formation of the electronic substrate may be high melting point metals such as molybdenum and tungsten or a noble metal such as gold. However, it is more desirable to use a conductor having a low electrical resistance and low cost, such as copper and alloys thereof.
Present state-of-the-art ceramic substrates are made from cordierite glass-ceramic particulate materials such as that disclosed in Kumar et al., U.S. Pat. No. 4,301,324. These substrates exhibit a dielectric constant of about 5 and a thermal coefficient of expansion (TCE) that closely matches that of silicon. It is desirable to fabricate substrates out of low dielectric constant materials so as to increase signal propagation speed, which varies inversely with the square root of the dielectric constant.
Prior to the cordierite glass-ceramic materials, alumina for a number of years had been an adequate dielectric material for microelectronic packaging. Alumina, however, has a dielectric constant approaching 10 which causes high signal propagation delay and low signal-to-noise ratio. Further, alumina has a TCE about twice as high as silicon which impacts the thermal fatigue resistance of the package. For low end applications, however, alumina (as well as other similar materials having a dielectric constant of about 10 and below) will be used for some time to come.
The present inventors, however, have discovered a vexing problem that is applicable to many multilayer ceramic materials and substrates fabricated therefrom.
It has been found by others that the vias do not completely seal to the ceramic material, thereby possibly resulting in a gap between the metallic via and the ceramic bulk material. This gap is undesirable as it reduces the hermeticity of the fabricated substrate as well as allowing fluids to seep into the substrate during processing. Accordingly, it has been proposed in Farooq et al., U.S. Pat. No. 5,073,180, the disclosure of which is incorporated by reference herein, to seal at least the top layer of a multilayer ceramic substrate with a composite via material consisting of metallic and ceramic (including glass) materials. The internal vias are essentially all metal. As taught by Siuta U.S. Pat. No. 4,594,181, the internal vias may also include small amounts of alumina or other ingredients to inhibit the densification of the metallic via.
What the present inventors have found is that at the interface between the composite sealing via and the metallic internal via, and/or at the interface between the internal via and the bulk ceramic, there is a mismatch of thermal coefficients of expansion and some difference in densification behavior during cofiring which makes the interface susceptible to fatigue failure when the substrate is exposed to thermal stress during post-firing processing. The result is that an unrepairable open may occur at one of the above interfaces. If the net containing the open cannot be rerouted, the entire substrate must be scrapped.
It is, therefore, a purpose of the present invention to have a substrate which does not suffer from such thermal fatigue-enhanced opens.
It is another purpose of the present invention to have a process for making such a substrate.
These and other purposes of the present invention will become more apparent after referring to the following description of the invention considered in conjunction with the accompanying drawings.